The present invention relates to the art of packaging semiconductor devices, and more particularly, to an improved silver-glass paste which is fired in an oven to securely bond semiconductor die to ceramic substrates.
The manufacture of large scale integrated (LSI) circuits has as its primary goal the lowest possible cost per electronic function performed. The fabrication processes used by the semiconductor industry are adopted with a view toward achieving this goal.
Masks and photolithography are used in building the LSI circuits on a silicon wafer. These circuits are probe-tested and individual dice bearing good circuits are selected from the sectioned wafer and assembled into packages. These packages are mounted and interconnected on printed circuit boards by "chip customers" in the manufacture of computers, communications equipment and defense electronics.
Advances in batch processing wafers simultaneously have led to dramatic reductions in fabrication costs in recent years. However, the costs of testing and packaging the LSI circuits are still very significant because each die must be handled individually. Therefore considerable effort is being applied to advancements in automatic attachment of the silicon die to their substrates. The bond between the silicon die and the ceramic substrate is critical to avoiding circuit failures. The substrate is used as a carrier to support the LSI circuit inside a dual-in-line or other package. Thermal shock, i.e. rapid, substantial changes in temperature may cause the bond to fail if it is unable to accommodate the different coefficients of thermal expansion between the ceramic and the silicon.
Silver-glass attachment pastes have been used in the past for attaching silicon die to ceramic substrates. One prior art silver-glass paste uses silver flakes and lead borate glass frit mixed in a liquid organic vehicle. During an initial drying phase, the liquid vehicle evaporates. Upon firing in an oven, the remaining organic vehicle decomposes and the glass melts down to wet the ceramic substrate, while the silver flake sinters together. Upon cooling the result is a bond between the silicon dice and the ceramic substrate.
The silver-glass die attachment paste is an alternative to gold-silicon eutectics, conductive epoxies and polyimides. The silver-glass paste does not have the special processing requirements of the gold-silicon eutectics, making it more adaptable to automation. The silver-glass paste is inorganic after a low-temperature burn-out, thereby eliminating moisture outgassing problems and thermal degradation associated with organic die attachment methods in hermetic packages. The fired silver-glass bond has a very good modulus of elasticity. Gold-silicon eutectics are not well suited for preventing failures with VSLI chip packages that undergo thermal shock because of their relatively poor modulus of elasticity.
U.S. Pat. No. 4,401,767 of Dietz et al. and U.S. Pat. No. 3,497,744 of Hornberger et al. disclose the use of silver-glass and other cermet materials for bonding silicon die to ceramic substrates. See also U.S. Pat. Nos. 4,459,166 and 4,436,785 of Dietz et al.